Humpback whales are suddenly hanging out in massive, densely packed “super groups” and scientists are trying to figure out why.

It’s a curious phenomenon that’s been spotted off the coast of South Africa by researchers and the general public.

Their meet-up spot is far away from where humpback whales usually summer in the Antarctic. The groups swell from 20 to 200 and they are focused on one thing — feeding.

“When you’re in a small boat with 200 humpback whales around you — they’re 14-meter animals — and you’ve got whales popping up all around you, it’s a really incredible experience,” lead researcher Ken Findlay told CNN.

He and his colleagues from South Africa’s Department of Environmental Affairs believe that these “super groups” feeding together is a relatively recent behavior, they wrote in PLOS One.

They observed the whales lunging, repeatedly diving and letting out the “pungent ‘fishy’ smell of whale blows.” Research cruises observed this behavior in 2011, 2014 and 2015 during the months of October and November, which are spring-summer months in South Africa.

The “super groups” seemed fluid, as some whales joined them but then left. Researchers observed that some whales traveled from afar to join the massive groups.

As the New Scientist pointed out, humpback whales aren’t known for being social. They’re usually spotted alone, in pairs or in small groups that don’t stay together long.

“Reasons for this recent novel behavior pattern remain speculative,” researchers wrote, but it could be due to increasing numbers of humpback whales in the region or the abundance of prey.

But researchers couldn’t identify what kind of prey prompted this massive meet-up.

“There is something a little mysterious about it, but it’s not mysterious that it’s happening, it’s the how it’s happening and the why it’s happening,” Findlay said. “We need time to unpack those questions.”

He said he and fellow researchers plan to publish a series of papers on the phenomenon. “I’ve been in humpback research since the late 1980s and we’ve never seen anything like this before.”

The spotting of so many super groups do mean something exciting for the state of these majestic creatures.

“What we found here is reflective of the abilities of these animals to bounce back from very severe whaling pressure in the last century. Over 200,000 humpback whales were taken from the Southern hemisphere last year,” Findlay said.

“It absolutely amazed me that 30 to 40 years later, we’ve seen these populations rebounding as they are. That gives me hope. It tells me something about the state of the ocean.”